This application relates generally to a device and method of gain calibration for a radiation detector. More specifically, this application relates to a device and method which allows a technician to efficiently set up a medical imaging gamma detector for gain calibration, start the calibration, and then proceed to address other tasks while the gain calibration automatically runs unattended.
In the field of Medical Imaging, one modality is nuclear medicine (gamma camera) imaging. This imaging can use a detector consisting of a scintillator backed by a plurality of either single anode photomultiplier tubes (PMTs) or multianode position sensitive PMTs (PSPMTs) with appropriate electronics. For brevity in the upcoming discussion, PMTs and/or PSPMTs will be referred to as PMTs, but anyone skilled in the art will recognize that either can be used with appropriate modifications. Furthermore, additional sensor types may become available in the future that can be used to replace or supplement PMTs.
In one application of such imaging, a patient is given a radioisotope either by injection or ingestion and then the detector(s), after being placed in close proximity to the patient, can determine where the radioisotope goes or has gone. Then, the device is used to detect the radioisotope as it travels through the patient.
The process of detection occurs when the radioisotope emits radiation, such as a gamma photon, for example, in the direction of the detector, and the photon is absorbed by a scintillator. The scintillator emits a flash of light (a scintilla) which is detected by one or more of the plurality of PMTs. The PMTs closer to the flash have a higher signal than those further away. By measuring the intensity of the flash at each PMT, and then using a centroid type calculation, a fairly accurate estimation of where the flash occurred is possible in a manner that is known in the art.
One of the chief requirements of high quality gamma camera imaging is good calibration of the detector(s) for accurate detection of the gamma photons. The typical calibration consists of, in order:                1. Offset calibration;        2. Gain Calibration;        3. Linearity and Energy calibration (in either order, different manufacturers have different preferences); and        4. Uniformity or flood calibration.        
The Offset calibration measures the quiescent output of the PMTs to allow its subtraction, which can be done in a manner that is known in the art. This allows the PMT signals to use the full range of the analog to digital converter (ADC).
Next is the gain calibration, which can be performed in a manner known in the art. To achieve high quality images, all the PMTs need to have the same output signal for the same input signal, i.e., they must have the same gain.
Next is either linearity then energy calibration or energy then linearity calibration, which can also accomplished in a manner that is known in the art. Linearity calibration measures the displacement of a calculated (detected) position from its actual position and creates a correction table (e.g., a look up table (LUT)). Energy calibration measures the energy peak at a multitude of detector positions and calculates a correction factor for each position so all peaks will be at the same channel.
Lastly, is the uniformity or flood calibration, which can be accomplished in a manner known in the art. This step typically involves flooding the entire detector input face with a uniform intensity of gamma rays. This is usually done by using a point source of radioisotope at some distance (usually 5 multiplied by the largest dimension of the detector). Any deviations from a uniform response in the images is typically corrected using a LUT of some type.
One of the problems of the above calibration techniques calibration is that they are typically time consuming and operator intensive. For instance, to do a traditional gain calibration, a technician will typically move a radioactive source collimated through a hole over the center of each of the PMTs in sequence. While the source is over a given PMT, its gain is calculated and adjusted. This can be very time consuming for the technician, who could be doing another task if the calibration could be automated.
Alternatively, some manufacturers simply flood the detector with a uniform intensity of gamma rays and try to pick appropriate signals as indicative of a given PMTs gain, as shown in U.S. Pat. No. 5,550,377, incorporated herein by reference. This is very difficult to do and can be prone to substantial undesirable errors.
Another group of methods involve using LEDs or other light references, as shown in U.S. Pat. Nos. 6,342,698; 6,087,656; 5,412,215; 5,237,173; 5,079,424, all incorporated herein by reference. These methods assume that the PMTs respond to the light sources in the same way as the scintilla, which is not generally true. U.S. Pat. No. 6,087,656, incorporated herein by reference, teaches the use of an ultraviolet source to excite the scintillation crystal so the PMTs are calibrated using the same light as detected during operation. While this may be an improvement over prior methods, there is still the problem of picking appropriate signals, as indicative of a given PMTs gain as in the flood technique.
It would be useful, for example, to allow a technician to efficiently set up a detector for gain calibration, start an automated gain calibration process, and then proceed to address other tasks while the gain calibration runs unattended.